1. Field of the Invention
The present invention relates to a power supply, and more particularly, to a switching mode power supply for supplying direct current power.
2. Description of the Related Art
A power supply for supplying power is used in all systems using electricity. The power supply must provide stable power. If the power supplied by the power supply is unstable, the system that receives the power may not operate properly. Therefore, providing stable power is an essential requirement of the power supply. In a switching mode power supply, switching means is used in order to supply stable power.
FIG. 1 is a circuit diagram of a conventional switching mode power supply. Referring to FIG. 1, the conventional switching mode power supply 101 includes a power source 191, an input controlling portion 111, a transformer 121, first and second diodes 131 and 132, first and second capacitors 141 and 142, first and second snubber circuits 151 and 152, a feedback signal generating portion 161, a feedback signal receiving portion 171, and a power switching integrated circuit 181. The power switching integrated circuit 181 is comprised of a comparator 183, a controlling portion 185, and a power transistor 187. The transformer 121 includes an input winding 123 and a feedback winding 125 in the primary windings and first and second windings 127 and 128 in the secondary.
An input voltage Vi is applied to the input winding 123 of the transformer 121 through the input controlling portion 111. When the power switch is turned on, current flows to the power transistor 187 through the input winding 123. When the power transistor 187 is turned off, voltage is generated in the first and second windings 127 and 128 and the feedback winding 125. Then, the first and second diodes 131 and 132 are turned on. When the first and second diodes 131 and 132 are turned on, the voltage generated in the first and second windings 127 and 128 is rectified through the first and second diodes 131 and 132, filtered by the first and second capacitors 141 and 142, and output as the output voltage Vo of the switching mode power supply 101.
The output voltage Vo is fed back to the feedback signal receiving portion 171 through the feedback signal generating portion 161. The feedback signal generating portion 161 converts the output voltage Vo into an optical signal and transmits it. The feedback signal receiving portion 171 receives the optical signal transmitted by the feedback signal generating portion 161, converts it into an electrical signal, and transmits it to the power switching integrated circuit 181.
The power switching integrated circuit 181 is comprised of a comparator 183, a controlling portion 185, and a power transistor 187. An NMOS transistor is used as the power transistor 187. The signal transmitted from the feedback signal receiving portion 171 is transmitted to the power transistor 187 through the controlling portion 185. When the output voltage Vo is higher than the voltage fed back from the power transistor 187, the power transistor 187 is turned off. When the power transistor 187 is turned off, the output voltage decreases. When the output voltage Vo is lower than the voltage fed back from the power transistor 187, the power transistor 187 is turned on and the output voltage increases.
A reference voltage Vr of 6.2 volts is applied to an inverting (-) input of the comparator 183. A synchronous voltage Vsync, in which a synchronous signal .PHI. sync input from the outside is added to a controlling voltage Vp output from the controlling portion 185, is applied to a non-inverting input. When the synchronous voltage Vsync is higher than the reference voltage Vr, the power transistor 187 is turned on. On the other hand, when the synchronous voltage Vsync is lower than the reference voltage Vr, the power transistor 187 is turned off. The waveform of the synchronous voltage Vsync is shown in FIG. 2.
FIG. 2 shows waveforms of signals input to and output from the power switching integrated circuit 181 shown in FIG. 1. Referring to FIG. 2, when a feedback winding voltage Ve generated between both ends of the feedback winding 125 is positive, the power transistor 187 is turned off. When the feedback winding voltage Ve is negative, the power transistor 187 is turned on. When the synchronous voltage Vsync is higher than the reference voltage Vr right after the power transistor 187 is turned off, a surge voltage 201 of about 500 volts is generated in the first and second diodes 131 and 132. When the surge voltage 201 is generated, the first and second diodes 131 and 132 may be broken, thus losing their rectification function.
The surge voltage 201 is generated as follows. When the level of the synchronous voltage Vsync exceeds that of the reference voltage Vr, the power transistor 187 is turned on. When the power transistor 187 is then turned off, voltage is generated in the first and second windings 127 and 128. Accordingly, the first and second diodes 131 and 132 become conductive. When the first and second diodes 131 and 132 are conductive, the current generated in the first and second windings 127 and 128 gradually decreases, flowing through the first and second diodes 131 and 132. When the power transistor 187 is turned on and then off before the current flowing through the first and second diodes 131 and 132 decreases enough, voltage is generated in the first and second windings 127 and 128. Accordingly, the surge voltage 201 is generated across the first and second diodes 131 and 132. The surge voltage 201 may severely damage the first and second diodes 131 and 132.
The conventional switching mode power supply 101 uses the first and second snubber circuits 151 and 152 in order to reduce the surge voltage 201. The first and second snubber circuits 151 and 152 are connected respectively to the first and second diodes 131 and 132, in parallel. The first and second snubber circuits 151 and 152 protect the first and second diodes 131 and 132 by absorbing the surge voltage 201 when the surge voltage 201 is generated across the first and second diodes 131 and 132.
The first snubber circuit 151 is constituted of a diode 151a, a capacitor 151b, and a resistor 151c. The first snubber circuit 151 and the second snubber circuit 152 have the same structure. The manufacturing expenses of the switching mode power supply 101 increase due to the first and second snubber circuits 151 and 152. To remove the need for the first and second snubber circuits 151 and 152, the first and second diodes 131 and 132 can be replaced by diodes having a short reverse recovery time. However, diodes having a short reverse recovery time are expensive, so this option is actually no cheaper If the generation of the surge voltage can be prevented, the first and second snubber circuits 151 and 152 and the diodes having a short reverse recovery time need not be used.